In U.S. Application No. 722,956, a hydraulic brake booster includes a piston which is movable in response to pressurized fluid in a pressure chamber to initiate a braking application. In order to conserve space and weight, the piston is hollow to substantially define a storage chamber within the piston. If the pressure of the pressurized fluid within the pressure chamber is insufficient to move the piston during braking, the storage chamber is communicated to the pressure chamber to assist in moving the piston, thereby providing a power assist to the braking application. Generally, the storage chamber is communicated to the pressure chamber when a spool valve within the brake booster fails to communicate pressurized fluid to the pressure chamber, either because of an inoperable pressure source or the spool valve becoming wedged in the brake booster housing.
With the pressure source rendered inoperable, the communication of pressurized fluid to the pressure chamber form the storage chamber is required to provide a power assist to braking in order to bring a vehicle to a stop. Consequently, a reliable valve member which operates to communicate the storage chamber within the piston with the pressure chamber is believed to be an improvement in the state of the art. Moreover, during assembly it is desirable to remove only valve members which are not satisfactory without scrapping any other parts of the hydraulic brake booster.